Laser doppler velocimeter

ABSTRACT

A laser doppler velocimeter for measuring one or more velocity components of a moving material, fluid or solid, using a single set of optical focusing elements. A unique two component beam splitter provides multiple pairs of parallel beams, one pair from each input radiation beam from a given laser source. 
     Each incident radiation input beam is separated by means of a selectively transmissive coating on one of the roof faces of an Amici prism into a first beam comprising substantially all of the polarized component parallel to the plane of incidence (P polarization) and one-half of the polarization component normal to the plane of incidence (S polarization), and a second beam comprising one-half of the S polarization component alone. At the measuring point, the two beams interfere, with the S components producing a differential doppler frequency superimposed on a non-doppler pedestal, while the P component contains only the non-doppler pedestal. The detector beam splitter directs the P polarization component to one radiation detector and the S polarization component to the remaining detector. The analog electrical equivalent signal to the optical P component signal is electrically subtracted from the electrical analog equivalent to the S component signal, the pedestal is removed, and this signal is divided by the P component signal to yield a signal having the pure doppler differential frequency information. 
     The invention provides dual beam redundancy, extremely simple alignment, and two velocity measuring ranges.

BACKGROUND OF THE INVENTION

This invention relates to an appartus for measuring the velocity ofmoving materials, e.g. gaseous, liquid or solid substances. Moreparticularly, this invention relates to an apparatus employing the laserdoppler effect for measuring the velocity of moving media.

Laser velocimeters are known which employ the doppler effect to measurethe velocity of moving fluid at a measuring point. Such laser dopplervelocimeters have the potential advantage over known mechanical devicesfor velocity component measurements of permitting such measurementswithout significantly disturbing the flow characteristics of the fluidbeing measured. Other advantages of laser velocimeters over mechanicaldevices are the relatively high speed of the measurement process and theability of such devices to perform measurements in relativelyinaccessible locations.

Although laser velocimeters using pulsed laser sources have beensuggested in the literature, known laser velocimeters typically employ acontinuous wave laser source. The coherence length of the lightgenerator in such a source is selected to be relatively long, e.g., ofthe order of several centimeters or more, in order to eliminate therequirement of exact path length equality of two related wavefronts fromsource to point of optical interference. In such systems, the use of ashort coherence length continuous wave or pulsed laser, e.g., aninjection laser diode, is precluded due to the fact that interference ofbeams from such a source cannot be predictably achieved due to the lackof substantially equal path lengths for related beams. Other limitationsinherent in known continuous wave laser velocimeters are the relativelylarge physical size necessitated by the relatively large lasers andassociated optics employed, and their relative instability in use, whichnecessitates frequent readjustment using relatively sophisticatedalignment techniques.

SUMMARY OF THE INVENTION

The invention comprises a pulsed laser velocimeter which is extremelycompact and rugged, which is relatively inexpensive and simple tofabricate, and which permits the measurement of one or more velocitycomponents of a moving material, fluid or solid, using a single set ofoptical focusing elements.

In the preferred embodiment, a unique two component beam splittercomprising four Amici roof prisms and four reflecting prisms providesmultiple pairs of parallel beams, one pair from each radiation beamincident to a different input face of the beam splitter, with thepolarization states of each beam in a pair being related in a uniquemanner. Each input beam is supplied by an injection laser diode operatedin a pulse mode. The parallel beams are focused by a focusing lens to ameasuring point along the optical axis of the laser velocimeter and theback reflected light from an object at the measuring point is focused bythe same focusing lens, compressed by a beam compression lens system anddirected to a detector beam splitter which divides the radiationincident thereto into two orthogonally polarized components, each ofwhich is directed to a separate radiation detector. Each detectorgenerates electrical output signals in response to the radiationincident thereto, which signals contain the doppler differentialfrequency information resulting from a moving object reflectingradiation from the interference fringes produced by the beam pair at themeasuring point.

By employing a pair of injection laser diodes positioned at differentinput faces to the two component beam splitter and operated in asequential pulsed manner, the velocity components in two directions canbe measured, and the resulting electrical signals can be processed toyield the magnitude and direction of the velocity vector of the objectunder measurement in the plane normal to the optical axis of the laserdoppler velocimeter at the measuring point. By using four pulsed lasersources each positioned at a different input face to the two-componentbeam splitter, two sets of redundant beam pairs are produced, eitherlaser of each set being capable of providing one velocity componentmeasurement.

Each Amici roof prism is provided with an optical coating over a portionof one of the roof faces. This coating, which preferably comprises asingle layer of titanium dioxide, transmits substantially all of thepolarized component parallel to the plane of incidence (P polarization)and equally divides the polarization component normal to the plane ofincidence (S polarization). At the measuring point, the two beamsinterfere, with the S components producing a differential dopplerfrequency superimposed on a non-doppler pedestal, while the P componentcontains only the non-doppler pedestal. The detector beam splitter isfabricated in such a manner that the P polarization component isdirected to one radiation detector and the S polarization component isdireted to the remaining detector. Thereafter, by subtracting the analogelectrical equivalent to the optical P component signal from theelectrical analog equivalent to the S component signal, the pedestal isremoved, and this signal is divided by the P component signal to yield asignal having the pure doppler differential frequency information.

The unique design of the two component beam splitter permits exactparallel alignment of emerging beams with the two component beamsplitter optical axis, relative angular orientation between the planesdefined by different emergent beam pairs resulting from incident laserbeams incident to different input faces of the Amici prisms, and pathlength equality for each emerging beam pair resulting from a laser beaminput at any input face, all by simple translations and rotations of theAmici prisms and their associated reflecting prisms. In addition,alignment of each of the laser diode sources relative to the twocomponent beam splitter is simply achieved by manipulating the laser andobserving that position at which the associated beam pairs intersectafter passing through the focusing lens. Since there is only onepossible intersection point located on the optical axis of the beamsplitter at the focus of the focusing lens, all pairs of intersectingbeams automatically intersect at the same point. Moreover, afteralignment of each laser diode relative to the two component beamsplitter, small off-axis movements of the focusing lens with respect tothe beam splitter do not alter the angle of intersection betweenassociated beam pairs and do not disrupt the actual intersection of thebeams.

The lateral spacing of the emergent beam pairs, and thus the angle ofintersection thereof at the measuring point, may be preselected as oneof two values by simply securing the associated reflecting prism to adifferent end face of the asociated Amici prism, and using the oppositeend face of the Amici prism as the radiation input face. Thus, twovelocity measuring ranges can be provided by the invention, either oneof which is predetermined when assembling the two component beamsplitter, and the same remaining optical elements may be employed.

For a fuller understanding of the nature and advantages of theinvention, reference should be had to the ensuing detailed descriptiontaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating a laser doppler velocimeteraccording to the invention;

FIG. 2 is an enlarged side elevation view showing the assembled twocomponent beam splitter;

FIG. 3 is an end elevation view of the two component beam splitter ofFIG. 2;

FIG. 4 is an exploded view illustrating the individual optical elementscomprising the two component beam splitter;

FIG. 5 is an end elevation view similar to FIG. 3 showing alternate beamgeneration; and

FIG. 6 is a block diagram of an electronic signal processing unit.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Turning now to the drawings, FIG. 1 illustrates a laser dopplervelocimeter constructed according to the invention. As seen in thisFIG., pulsed coherent light from a laser source 11 is collected by acollimating lens 12 and directed onto an input face 14 of a noveltwo-component beam splitter generally designated by reference 15 andillustrated in FIGS. 2-5. Pulsed laser source 11 is preferably aninjection laser diode, e.g., a type SG 2007 laser diode available fromRCA Corporation. Collimating lens 12 is preferably a cemented doubletwhich is diffraction limited over a field angle containing the laserdiode emitting region.

Two component beam splitter 15 divides the incident light beam 13 into apair of parallel beams 20, 21 which travel directly to a focusing lens23 which focuses beams 20, 21 to a measuring point M located on theoptical axis of the system at the focus of lens 23. Focusing lens 23 ispreferably an air spaced triplet which is diffraction limited over thebeam regions. Measuring point M is located in the path of a material,e.g., a gas, liquid or solid, whose velocity is to be measured, so thatthe back reflected light represented by a single ray 25 contains thedoppler differential frequency information in optical form. The backreflected light is collected and collimated by focusing lens 23,compressed in diameter by a two element beam compressing lens assemblygenerally designated by reference numeral 26 (which may comprise a pairof air spaced triplets), and directed onto the input face of a detectorbeam splitter 30.

The compressed beam 29 is separated into two beams of orthongonalpolarization by a detector beam splitter 30, which preferably comprisestwo 45° prisms, one of which is provided with a dielectric polarizingbeam splitter coating on the diagonal face, the prisms being cementedtogether at their mutual diagonal interfaces with optical cement. Beams31, 32 are imaged by a pair of detector lenses 34, 35, which maycomprise air spaced triplets, onto the light input of a pair of lightsensitive detectors 36, 37. Detectors 36, 37 preferably compriseconventional avalanche photodiode detectors such as two type TIXL 69detectors sold by Texas Instruments, Inc. Although beam 31 and theopticl axis of lens 34 are illustrated as lying in the plane of thepaper in FIG. 1, it should be noted that this representation is forconvenience only. In actuality, beam splitter 30, lens 34 and detector36 are all positioned at a different rotational angle about the opticalaxis of the system of FIG. 1 so that beam 31 emerges at an angle to theplane of the paper, e.g. ± 45°.

The electrical output signals on terminals 38, 39 of detectors 36, 37are coupled to the electronic signal processing system shown in FIG. 6and described below for interpreting the differential frequencyinformation and determining the velocity of the medium at measuringpoint M.

With reference to FIGS. 2-4, two component beam splitter 15 is comprisedof two prism types: viz. four Amici prisms 41-44 and four reflectingprisms 45-48 each in contact with a different one of Amici prisms 41-44,respectively. As illustrated for Amici prisms 42 and 43, one of the rooffaces 57 of each Amici prism 41-44 is provided with a beam splittingcoating 49 responsive to incident monochromatic light for transmittingsubstantially all of the P polarization component of the lighttherethrough while splitting substantially equally the S polarizationcomponent of the incident light. Coating 49 preferably comprises asingle layer of titanium dioxide; however, other equivalent coatingmaterials known to those skilled in the art may be employed.

Each of the reflecting prisms 45-48 is provided with a conventionalmetallic reflecting coating 52 on the rear surface 62 thereof having ahigh reflectivity coefficient.

Each Amici prism has a pair of end faces 55, 56, either of which mayserve as the light input or light output face as described more fullybelow, and a pair of roof faces 57, 58, to the former of which thecoating 49 is applied.

The eight optical elements 41-48 comprising the two component beamsplitter 15 are permanently secured together after the alignmentprocedure described below by means of conventional optical cement whichis transparent to monochromatic light of the wave length produced byinjection laser diode 11. Preferably an ultraviolet setting cement suchas Summers Laboratories, Inc. type UV71 is used; however, otherequivalent cement may be employed, as desired.

FIG. 3 illustrates the relative position of emergent beams 20, 21resulting from an incident input beam 13 to input face 55 of Amici prism41 of the two component beam splitter assembly 15 in the preferred modeof operation. As seen in this FIG., beam 20 emerges from surface 63 ofreflecting prism 46, while beam 21 emerges from surface 63 of reflectingprism 47. The paths of the rays through the two component beam splitterassembly are as follows. Beam 13 enters Amici prism 41 substantiallynormal to surface 55 in the region overlying the coated surface 49,strikes coated surface 49 and is split into a first reflected beamcomprising one half of the S polarization component and a secondtransmitted beam comprising the remaining one half of the S polarizationcomponent and the entire P polarization component of the incident beam13. the reflected beam exits Amici prism 41 via uncoated roof face 58,enters Amici prism 42 via roof face 57 and exits via end face 56. Thebeam exiting from end face 56 enters reflecting prism 46 via face 61, istotally internally reflected at face 63, is reflected by face 62 andexits via face 63 as beam 20. The transmitted beam enters Amici prism 43via uncoated roof face 58, exits via face 56, enters reflecting prism 47via input face 61, is totally internally reflected at exit face 63, isreflected at face 62 and emerges as beam 21 from face 63.

FIG. 5 illustrates the relative position of emergent beams 20', 21resulting from an incident input beam 13 to a different portion of inputface 55 of Amici prism 41. As seen in this FIG., beam 20' emerges fromsurface 63 of reflecting prism 48, while beam 21 emerges from surface 63of reflecting prism 48, while beam 21 emerges from surface 63 ofreflecting prism 47 as above. The alternative paths of the rays throughthe two component beam splitter assembly 15 are as follows. Beam 13enters Amici prism 41 substantially normal to surface 55 in the regionoverlying coated surface 49 of Amici prism 43 and exits via coated roof58. The exiting beam strikes coated surface 49 of Amici prism 42 and issplit into a first reflected beam comprising one-half of the Spolarization component and a second transmitted beam comprising theremaining one-half of the S polarization component and the entire Ppolarization component of the incident beam 13. The reflected beamenters Amici prism 44 via roof face 58 and exits via end face 56. Thebeam exiting from end face 56 enters reflecting prism 48 via face 61, istotally internally reflected at face 63, is reflected by face 62 andexits via face 63 as beam 20'. The transmitted beam enters Amici prism42 via roof face 57, exits via face 58, enters Amici prism 43 via theuncoated portion of roof face 57, and exits from Amici prism 43 via endface 56. The exiting beam from Amici prism 43 enters reflecting prism 47via input face 61, is totally internally reflected at exit face 63, isreflected at face 62 and emerges as beam 21 from face 63. It should benoted that, with either the FIG. 3 or the FIG. 5 arrangement, theemergent beam pair 20 (20'), 21 lies in a plane which is offset from thegeometrical axis of the beam splitter assembly 15.

Although only one pulsed laser source 11 is illustrated in full line inFIG. 1, and only one input beam 13 is shown in FIGS. 1 and 2, inpractice four laser sources 11 are employed, with each such additionallaser source 11 and associated collimating lens 12 being arrangedadjacent the input face 55 of a different Amici prism 42-44. One suchadditional pulsed laser source 11 is illustrated in broken lines in FIG.1 with the associated collimating lens 12, both elements beingpositioned to direct a beam 13' toward input face 55 of Amici prism 43.

In operation, the plurality of laser sources 11 are sequentiallyoperated to serially generate different resulting beam pairs. Due to thesymmetry of the two component beam splitter 15 about the geometricalaxis thereof, the path of the other input beams and the resulting twocomponent beams through the various elements 41-48 of the assembly 15when an input beam is incident on the input face 55 of a different Amiciprism will be apparent to those skilled in the art. Each emergent beampair is focused by the same lens 23 onto the measuring point M. Eachpair of beams permits measurement of a component of velocity of thematerial being measured along a line formed by the intersection of thebeam pair plane and a plane normal to the axis of the beam splitter 15and passing through the measuring point M. Thus, two beam pairs providea measurement of two components of velocity in the latter plane. Therelative orientation of the two components may be spatially adjusted inthe manner described below. It is noted that there are four possiblebeam pairs which may be generated, each pair resulting from the beamincident to the light input face of a different Amici prism 41-44.

As illustrated in the FIGS., face 55 of each of the Amici prisms 41-44serves as the light input face, while reflecting prisms 45-48 arecemented to face 56 of these elements. With this configuration, thelinear separation distance between the emergent beam pairs, e.g., beams20, 21, can be adjusted over a first range of values in the mannerdescribed below. If a different range of values of smaller magnitude isdesired in a given application, the linear beam spacing may be narrowedby cementing reflecting prisms 45-48 onto faces 55 of prisms 41-44, andfaces 56 of these latter elements may serve as the light input face.Since the diameter of an imaginary circle through which the emergingbeam pair center lines pass is identical in this alternateconfiguration, optical elements 23, 26, 30, 34, and 35 need not bealtered. As will be appreciated by those skilled in the art, the angleof intersection of beams 20, 21 at measuring point M determines themagnitude of the doppler frequency for a given velocity at M.

In general, the greater the angle between the convergent beams, thesmaller the fringe spacing, and the smaller the fringe spacing, thehigher the doppler frequency for a given velocity of the material at themeasuring point M. Thus, the invention can be tailored to two differentvelocity ranges by simply reversing the arrangement of the beam splitterassembly 15 components.

The beam splitter assembly 15 is adjusted during assembly in thefollowing manner. In order to insure exact parallelism of an emergentbeam pair, e.g., beams 20, 21, to the optical axis of the beam splitterassembly 15, the input beam 13 is directed onto the input face and theappropriate pair of reflecting prisms 45-48 are rotated about an axisnormal to the exit face 56 of the associated Amici prism until the beamsare parallel. This step is repeated for each laser source 11 and eachpair of reflecting prisms from the faces 63 of which the associatedbeams comprising a given beam pair emerge. Once all reflecting prisms45-48 are fixed, there is only one angular orientation of each laserrelative to the associated light input face 55 for which the emergingbeams are parallel.

Next, relative angular orientation between planes defined by respectivepairs of emergent beams, each resulting from an input beam at adifferent one of the Amici prism input faces 55, is achieved bytranslating the individual reflecting prisms 45-48 along the respectivecontact faces 56 of the associated Amici prisms 41-44.

Thereafter, the path lengths of respective beam pairs from an input atany given Amici prism face 55 are equalized by translating each Amiciprism 41-44 and its associated reflecting prism 45-48 with respect tothe remaining Amici and reflecting prisms along the beam splitteroptical axis.

Once the above adjustments have been made, the beam splitter assembly 15is mounted in a housing (not shown) together with focusing lens 23 andelements 26, 34 and 35. In the preferred embodiment, utilizing laserinjection diode sources 11, the housing comprises a cylindrical memberhaving a diameter of about four inches and an axial length of about teninches. Thereafter, the individual lasers 11 are aligned with respect tothe beam splitter assembly by arranging the beam from a given laser 11normal to the associated Amici prism light input face, preferably byautocollimating the beam, and repositioning the laser 11 to vary theincident beam angle until the resulting beam pair intersects afterpassing through focusing lens 23. It is noted that the only possiblepoint of exact intersection of a pair of beams is precisely at the focusof focusing lens 23. Thus, once the beam splitter is correctly assembledeach pair of intersecting beams automatically intersects at the samemeasuring point M. Moreover, once laser sources 11 are all aligned withrespect to the beam splitter assembly 15, small relative movements ofthe focusing lens 23 off the geometrical axis of the assembly do notaffect either the fact of intersection of all beams or the angle atwhich the beam pairs intersect, but merely the spatial location ofmeasuring point M. While movements of up to ten times the blur diameterof lens 23 may be tolerated, preferably any such movements should beconfined to a magnitude no greater than the blur diameter of lens 23. Inone such lens 23 incorporated in a working embodiment of the invention,the worst case blur diameter is 0.020 inch. This arrangement providesthe advantage that slight misalignment of focusing lens 23 does notadversely affect the operation of the invention.

A key feature of the invention is the manner in which the image of thelaser source 11 is reflected in passing through the beam splitterassembly 15. It is known that the emitting region of many laser sources,such as an injection laser diode, is essentially a line segment, whichmay be defined as an emitting region having a length to width ratio onthe order of 100 or more. It has been found experimentally thatreversing the image of a line segment emitting region about thelongitudinal axis of the line segment when producing a pair of beamsdoes not impair the capacity of the two images in the beams to interfereoptically when superimposed. However, reversing the images about anyother axis destroys coherence between superimposed images due to thefact that light radiation from one end of the line segment issubstantially incoherent with respect to the light radiation from theother end of the same line segment. Thus, when an image reversed aboutany other axis than the longitudinal axis of that line segment issuperimposed on the original image, coherent optical interference doesnot result.

This problem has been solved in the invention by insuring that eachimage pair of the laser source formed at the measuring point M comprisesa pair of mirror images, i.e. one image is reversed about thelongitudinal axis of the laser emitting region, and by subjecting thetwo emerging beams to the same number of turnings by reflection aboutthe other two axes.

An important aspect of the invention is the manner in which thedifferential doppler frequency information is isolated from thesuperimposed non-doppler noise information in back-reflected beam 25. Asnoted above, coating 49 on surfaces 57 of the Amici prisms 41-44transmits substantially all of the P component of polarizationtherethrough while splitting equally the S polarization component of theincident beam. The emergent beam 21 thus contains all of the Ppolarization component and one half of the S polarization component ofthe input beam 13, while beam 20 comprises the remaining half of the Spolarization component of the input beam 13. Both beams 20, 21 areintensity modulated with time in accordance with any fluctuations in theoutput of laser source 11. When beams 20, 21 intersect at measuringpoint M only the common S components thereof interfere to produce thedifferential frequency shift, i.e., the doppler differential frequencyinformation, since only identical polarization states can interfere.After reflection at point M, the S polarization component of thereflected beam represented by ray 25 contains the doppler differentialfrequency information superimposed on the intensity modulation due tothe source 11 fluctuations and also variations in the scatteringcapability of the medium under measurement, while the P polarizationcomponent contains only the intensity modulation information. Thismodulation information is normally considered to be background noisewhich masks the doppler differential frequency information and does notcontribute information to the velocity measurement. The returning beam25, as noted above, is separated by detector beam splitter 30 into twocomponents: viz. the S component which is directed to detector 36, andthe P component which is directed to detector 37. With reference to FIG.6, the resulting electrical output signal on conductors 38, 39 which arethe electrical analogs of the S and P polarization components,respectively, of the back reflected light are then coupled to thedifferential inputs of a conventional electrical subtracting circuit 70,the output of which is coupled to the input of a first logarithmicamplifier 72. Conductor 39 is also coupled to the input of a secondlogarithmic amplifier 74. The outputs of the logarithmic amplifiers 72,74 are coupled to the differential inputs of a second electricalsubtracting circuit 77. The output of circuit 77 is coupled to the inputof zero crossing detector 78 which generates a pulse for each zerocrossing of the analog input signal in the positive direction. Theoutput of zero crossing detector 78 is coupled to the input of a counter79 which counts the number of pulses in a given burst of receivedradiation, which count represents the differential doppler frequency.The output of counter 79 is coupled to a conventional display circuit80, which may comprise a multicharacter digital light emitting diodearray or the like.

As noted above, the output signal from S polarization detector 36comprises an electrical signal which is the analog equivalent of the Slight component of the back reflected beam 25, while the output of the Ppolarization detector 37 comprises the electrical analog to the Ppolarization component of beam 25. The S signal component, which has awaveform suggested by waveform 90 contains the differential dopplerfrequency information superimposed on a pedestal term, which can berepresented by the following equation:

    I.sub.s = [1+m.sup.. cos (2πf.sub.d t) ]p (t)

where I_(s) is the S polarization intensity, m is the modulationfraction, f_(d) is the doppler frequency and p(t) is the pedestal term.

The P term suggested by waveform 91 contains only the pedestal termwhich can be represented by the following equation:

    I.sub.p = k.sup.. p(t), k a constant

By adjusting the relative gains of the two detectors 36, 37 in aconventional fashion, the pedestal terms p(t) can be equalized. Whenwaveform 91 is subtracted from waveform 90 in circuit 70, the resultingsignal suggested by waveform 92 comprises a 100% doppler frequencymodulated pedestal term. When this signal is divided by the pedestalterm p(t) by logarithmic amplifiers 72, 74 and electrical subtractioncircuit 77, the resulting signal suggested by waveform 93 is the puredoppler frequency given by the following equation:

    log [m.sup.. cos (2πf.sub.d t) ]=log (m)+ log [cos (2πf.sub.d t) ]

Since this signal contains only the doppler frequency information, therelatively simple frequency counter and display circuit comprisingelements 78-80 may be employed to yield the desired information.

Another significant aspect of the invention is the redundancy of themeasuring beam pairs when four lasers are employed. The laser inputbeams entering Amici prisms 41 and 43 produce redundant measurements ofone velocity component, while beams entering Amici prisms 42 and 44produce redundant measurements of the second velocity component. Thus,if focusing lens 23 becomes contaminated in a region thru which one beamof a given pair passes, e.g., by dirt, bug spots, or the like, theredundant beam pair, consisting of two beams which pass through theother regions of lens 23, is still available to provide an accuratemeasurement of the same velocity component.

As will now be apparent, laser doppler velocimeters fabricated inaccordance with the invention provide a number of distinct advantagesover known devices. The beam splitter assembly 15, for example, can befabricated from two sets of identical optical elements of relativelysimple configuration. Further, due to the fact that the two componentbeam splitter assembly 15 may be adjusted to provide exact path lengthequality for each beam pair, pulsed laser sources having very shortcoherence lengths, i.e., lengths on the order of microns, may beemployed. In addition, emergent beam adjustment is relatively simple toperform, as is relative adjustment of lasers 11, focussing lens 23 andoptical elements 26, 30, 34 and 35. Moreover, due to the spacing of theemergent beams off the optical axis of the complete device, the beamcompression lens assembly 26, detector beam splitter 30 and detectorlenses 34, 35 may all be positioned within the housing containing thedevice. Further the rigid construction of the beam splitter assembly 15,the components of which are cemented together after initial adjustment,renders devices constructed according to the teachings of the inventionsubstantially insensitive to mechanical shocks and vibrations.

The compactness, ruggedness, and durability of the invention renders thedevice suitable in a wide variety of applications. In addition to theordinary uses to which anemometers have been applied in measuring fluidflow, e.g., wind tunnels, water flowing in a conduit, and flameprocesses, the invention may also be employed in those applicationsrequiring highly accurate velocity measurements of solid objects. Thedevice may be used for example to measure the velocity of filamentsextruded from a processing fixture or die in paper mills, wire drawingoperations, and generally in any application which requires the accuratemeasurement of the velocity of an object or medium without physicalcontact with the medium, or which requires velocity measurement in arelatively inaccessible location to which the beam pairs can befocussed.

Although beam splitting coating 49 has been specifically disclosed ashaving the property of equally dividing the S polarization component ofthe incident light, it should be noted that this is not an absoluterequirement for the proper operation of devices constructed according tothe teachings of the invention. Although this mode is preferred, theinvention envisions other fractional divisions of the S polarizationcomponent if deemed useful or desirable in any application of theinvention.

While the above provides a full and complete disclosure of the preferredembodiments of the invention, various modifications, alternateconstructions and equivalents may be employed without departing from thetrue spirit and scope of the invention. Therefore the above descriptionand illustrations should not be construed as limiting the scope of theinvention which is defined by the appended claims.

What is claimed is:
 1. A laser doppler velocimeter for measuring thevelocity of material at a reference point, said velocimeter comprising:apulsed laser source for generating a pulsed coherent radiation inputbeam having first and second orthogonal polarization components; firstbeam splitter means for generating a pair of parallel emergent beams ofcoherent radiation from said input beam, said parallel emergent beamshaving different polarization states; said beam splitter means includinga first optical axis, a radiation input face to which said input beam isdirected, a pair of radiation output faces, said beams emerging fromsaid radiation output faces parallel to said first optical axis, meansfor generating a first one of said pair of parallel emergent beams as apreselected fractional portion of said first polarization component andfor generating the remaining one of said pair of parallel emergent beamsas substantially all of said second polarization component and theremaining fractional portion of said first polarization component; meansfor focussing said pair of beams to said reference point to producereflected radiation having a first polarization component containingboth doppler differential frequency information and background noise anda second polarization component containing substantially only backgroundnoise; and means for separating said reflected radiation into a pair ofbeams containing said first and second polarization components,respectively, so that the separated reflected beam pair may be directedto a measurement device.
 2. The combination of claim 1 wherein saidpulsed laser source comprises a laser injection diode.
 3. Thecombination of claim 1 including a plurality of sequentially operablepulsed laser sources for generating a plurality of sequentiallyappearing pulsed coherent radiation input beams;and wherein said firstbeam splitter means includes a plurality of radiation input faces towhich said plurality of input beams are directed, each input beam beingdirected to a different one of said radiation input faces, and aplurality of pairs of radiation output faces.
 4. The combination ofclaim 1 wherein each said fractional portion is substantially one-half.5. The combination of claim 1 wherein said focusing means includes meansfor focusing said reflected radiation beam toward said separating means.6. The combination of claim 1 wherein said separating means comprisessecond beam splitter means for separating said reflected radiation beaminto a first beam containing said first reflected beam polarizationcomponent and a second beam containing said second reflected beampolarization component.
 7. The combination of claim 1 wherein saidpulsed laser source has an emitting region which comprises a linesegment having a longitudinal axis and a pair of mutually orthogonaltransverse axes, and wherein said first beam splitter means includesmeans for independently rotating the images of said line segment alongfirst and second optical paths within said first beam splitter means,the difference between the number of rotations of said images about saidtransverse axes along said first and second paths comprising an eveninteger and the difference between the number of rotations of saidimages about said longitudinal axis comprising an odd integer.
 8. Thecombination of claim 1 wherein said beam splitter means includes meansfor providing equal path lengths therewithin for said pair of emergentbeams.
 9. The combination of claim 1 further including first and seconddetector means each having a radiation input face in the path of adifferent one of said pair of beams emerging from said separating meansfor providing electrical signals equivalent to said first and secondpolarization components of said reflected beam pair, means coupled tosaid detector means for subtracting one of said polarization componentsignals from the other of said polarization component signals, meanscoupled to said subtracting means for dividing the electrical outputsignal therefrom by said one of said polarization components, and meanscoupled to the output of said dividing means for measuring the frequencyof the electrical output signal therefrom.
 10. The combination of claim9 wherein said dividing means comprises first and second logarithmicamplifying circuits each having an output, and an electrical subtractioncircuit having a pair of inputs each coupled to the output of adifferent one of said logarithmic amplifiers.
 11. The combination ofclaim 1 wherein said input beam generated by said laser source comprisesnon-plane polarized radiation.
 12. A method for isolating differentialdoppler frequency information from background noise in a laser dopplervelocimeter, said method comprising the steps of:a. generating acoherent radiation input beam composed of mutually orthogonalpolarization components; b. deriving from said input radiation beam afirst probe radiation beam comprising a predetermined fractional portionof one of said mutually orthogonal polarization components and a secondprobe radiation beam comprising the remaining fractional portion of saidone of said mutually orthogonal polarization components andsubstantially all of the other one of said mutually orthogonal radiationcomponents; c. focussing said first and second probe beams to ameasuring point to produce a reflected beam having a first orthogonalpolarization component containing both said differential dopplerfrequency information and said background noise and a second orthogonalpolarization component containing said background noise alone; d.separating said reflected beam into said mutually orthogonal componentsthereof; and e. combining said mutually orthogonal components of saidreflected beam to cancel said background noise.
 13. The method of claim12 wherein each said fractional portion is substantially one-half. 14.The method of claim 12 wherein said step (b) of deriving includes thestep of directing said input beam onto a radiation beam splittingsurface which is partially reflective to said one of said mutuallyorthogonal polarization components and substantially totallytransmissive to said other one of said mutually orthogonal polarizationcomponents so that said first probe radiation beam comprises saidpredetermined fractional portion of the polarization component of saidinput beam normal to the plane of incidence of said beam splittingsurface and said second probe radiation beam comprises said remainingfractional portion thereof and substantially all of the polarizationcomponent of said input beam parallel to the plane of incidence of saidbeam splitting surface.
 15. The method of claim 12 wherein said step (b)of deriving includes the step of directing coherent radiation input beamonto a beam splitting polarization medium.
 16. The method of claim 12wherein said step (b) of deriving includes the step of producing saidfirst and second probe radiation beams in mutually parallel fashion. 17.The method of claim 12 wherein said step (e) of combining includes thesteps of (i) generating first and second electrical signals which arethe electrical analogs of said mutually orthogonal polarizationcomponents of said reflected beam and (ii) subtracting said first andsecond electrical signals to cancel said background noise.
 18. Themethod of claim 17 wherein said step (e) of combining further includesthe steps of (iii) dividing the signal resulting from said step (ii) ofsubtracting by one of said first and second electrical signals.
 19. Asystem for isolating differential doppler frequency information frombackground noise in a laser doppler velocimeter, said systemcomprising:means for generating a coherent radiation input beam composedof mutually orthogonal polarization components; means for deriving fromsaid input radiation beam a first probe radiation beam comprising apredetermined fractional portion of one of said mutually orthogonalpolarization components and a second probe radiation beam comprising theremaining fractional portion of said one of said mutually orthogonalpolarization components and substantially all of the other one of saidmutually orthogonal radiation components; means for focussing said firstand second probe beams to a measuring point to produce a reflected beamhaving a first orthogonal polarization component containing both saiddifferential doppler frequency information and said background noise anda second orthogonal polarization component containing said backgroundnoise alone; means for separating said reflected beam into said mutuallyorthogonal components thereof; and means for combining said mutuallyorthogonal components of said reflected beam to cancel said backgroundnoise.
 20. The combination of claim 19 wherein each said fractionalportion is substantially one-half.
 21. The combination of claim 19wherein said means for deriving includes a radiation beam splittingsurface which is partially reflective to said one of said mutuallyorthogonal polarization components and substantially totallytransmissive to said other one of said mutually orthogonal polarizationcomponents so that said first probe radiation beam comprises saidpredetermined fractional portion of the polarization component of saidinput beam normal to the plane of incidence of said beam splittingsurface and said second probe radiation beam comprises said remainingfractional portion thereof and substantially all of the polarizationcomponent of said input beam parallel to the plane of incidence of saidbeam splitting surface.
 22. The combination of claim 19 wherein saidmeans for deriving includes a beam splitting polarization medium andmeans for directing said coherent radiation input beam onto said medium.23. The combination of claim 19 wherein said means for deriving includesmeans for producing said first and second probe radiation beams inmutually parallel fashion.
 24. The combination of claim 19 wherein saidmeans for combining includes means for generating first and secondelectrical signals which are the electrical analogs of said mutuallyorthogonal polarization components of said reflected beam, and means forsubtracting said first and second electrical signals to cancel saidbackground noise.
 25. The combination of claim 24 wherein said means forcombining further includes means for dividing the signal resulting fromsaid means for subtracting by one of said first and second electricalsignals.